Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly/idempotent/closed-world, so the safety bar is low. The description adds genuinely useful behavior beyond that: the rounding convention (half-up cents), the interest-only alternative payment path, the Fannie Mae PITIA composition, the four lender bands, and the dual JSON + plain-summary + link return. It does not describe defaults or missing-cost behavior, which matters because several cost inputs default to 0 but lenders 'always include' them.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the formula, then computation detail, then output shape — a logical order with no filler. The middle sentence is long and heavily compounded (formula, compounding, rounding, PITIA components, bands, sourcing note all in one), which costs some scannability.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With 12 parameters, no output schema, and the return format explained (JSON + summary + link), the description covers enough to call the tool correctly. The main gap is that it never says how defaults behave — e.g. whether omitted tax/insurance are silently treated as zero and how that shifts the ratio.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and every parameter carries a description, so the schema does the heavy lifting. The description contributes only indirect semantics (what PITIA aggregates, that rent is 'qualifying' rent) without explaining params themselves, so baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

Names a specific verb and resource and immediately supplies the formula it implements (qualifying rent / PITIA), plus the exact computation path (annuity formula, monthly compounding) and the output shape. An agent can tell this is the core ratio calculator rather than the sibling solvers (max_loan_for_target_dscr, rent_needed_for_target_dscr) that invert the same formula.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage is implied by the input model (rent + PITIA costs in, ratio out), and the note that lender minimums vary gives context for interpreting results. However, the description never states when to call this versus the inverse siblings or compare_scenarios, and gives no exclusions or prerequisites.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.